Various types of medical devices are employed to monitor electrical or other activity of the heart and to provide therapy to the heart in response to the detection of irregular cardiac rhythms. Such devices may be implantable beneath the skin of a patient, i.e., in the patient's chest. Such implantable devices include a hermetically sealed canister containing electronic circuitry for implementing the functions of the device, one or more electrodes implanted in one or more of the ventricles or atria of the heart, or in close proximity thereto, and leads for connecting the electrodes to the circuitry within the device canister. The device circuitry includes circuitry for detecting electrical signals produced by the heart, which signals are picked up at the electrodes, along with circuitry, typically implemented in a microprocessor, for analyzing the thus detected cardiac signals. The device may also include circuitry for providing therapy in the form of electrical signals applied to the heart. Such signals are provided to the heart, via the leads and electrodes mounted in the heart, in response to the detection of an irregular cardiac rhythm by the analysis circuitry based on the detected cardiac activity signals. The implantable device may also include a transmitter/receiver, for transmitting cardiac activity and other information to an external device for, e.g., storage and/or further analysis, and for receiving information, such as programming instructions, from the external device via, for example, an RF link.
An example of such an implantable cardiac device is a bradycardia pacemaker. A bradycardia pacemaker provides relatively low level electrical pulses to the heart to stimulate heart activity when the natural cardiac rate provided by the heart is too low. A dual chamber bradycardia pacemaker includes electrodes positioned in both the atria and ventricles of the heart for detecting naturally occurring atrial and ventricular activations and for providing pacing pulses to the atria and/or ventricles as needed. Such a device monitors the time between sensed and paced atrial and ventricular activations and provides pacing pulses as needed to maintain an adequate heart rate. For example, such a device will note the occurrence of a sensed or paced atrial or ventricular event and, if a subsequent naturally occurring atrial and/or ventricular event is not sensed within a certain time (escape interval) following the first sensed or paced event, a pacing pulse will be applied to the atria and/or ventricles to maintain a desired heart rate.
Although the primary purpose of a bradycardia pacemaker is to provide therapy for heart rates which are too slow, such pacemakers are also typically programmed to deal with naturally occurring atrial and ventricular rates which are too rapid. For example, such a device may detect a too-rapid ventricular rate (ventricular tachycardia). Antitachycardia pacing may be provided to the ventricles to attempt to terminate the ventricular tachycardia. Antitachycardia pacing typically involves a train of pacing pulses applied to the ventricles at a rate slightly higher than the rate of the tachycardia.
In many cases, a bradycardia pacemaker will be programmed to pace the ventricles of the heart so as to track the naturally occurring atrial rate. This is effective in many patients wherein the natural pacemaker which initiates a cardiac cycle in the atria performs normally most of the time. In such a case, the naturally occurring atrial rate is representative of the metabolic demand of the patient (e.g., the atrial rate will increase during exercise or other activity, and decrease during rest periods). Thus, pacing the ventricles to track the atrial rate in such a case ensures an adequate pacing rate for the patient's metabolic demand. However, if the naturally occurring atrial rate becomes too rapid, i.e., a pathologic atrial rhythm (atrial tachycardia) occurs, too rapid ventricular pacing may result if the pacemaker continues to track the atrial rate. Typical bradycardia pacemakers are programmed to deal with such supraventricular rhythm disturbances by automatically switching from an atrial tracking mode of operation (e.g., DDD or DDDR mode) to a mode of operation which does not track the atrial rate (e.g., VVI or VVIR mode) when the detected atrial rate exceeds a maximum atrial tracking rate or another pathological atrial rhythm is sensed by the pacemaker which may otherwise result in too rapid ventricular pacing.
Another type of implantable cardiac device is a defibrillator. A defibrillator detects for very rapid irregular rhythms (fibrillation) in the atria or ventricles. In response to the detection of fibrillation, the defibrillator provides a relatively high energy electrical defibrillation shock to the chambers of the heart which are in fibrillation to attempt to defibrillate the heart to return to a normal sinus rhythm. Rapid detection and defibrillation of ventricular fibrillation is critical. During ventricular fibrillation no blood is circulated by the heart, therefore, rapid defibrillation is required to prevent death. Atrial fibrillation is typically treated by a relatively high energy level shock provided to the atria in synchronism with a ventricular activation (atrial cardioversion). The atrial shock is provided in synchronism with a ventricular depolarization to prevent the attempted atrial cardioversion from inducing a more serious ventricular arrhythmia.
Many modern implantable cardiac devices combine the features of a cardiac pacemaker with those of a cardiac defibrillator. Such devices provide bradycardia pacing in a normal manner, along with monitoring of the ventricles and/or atria to detect for occurrences of ventricular or atrial fibrillation. If ventricular or atrial fibrillation is detected, defibrillating or cardioverting electrical energy is applied to the ventricles or atria in a conventional manner in an attempt to restore a more stable cardiac rhythm. Such devices may incorporate algorithms for distinguishing between, e.g., different rates of ventricular tachycardia, which may be pacing terminable, e.g., via antitachycardia pacing, and ventricular fibrillation, which is not pacing terminable, i.e., requires defibrillation. An example of such an algorithm is described, for example, in U.S. Pat. No. 5,342,402 to Olson, et al. Currently pending U.S. patent application Ser. No. 09/343,924, filed Jun. 30, 1999, by Milton M. Morris, and entitled Therapy-Selection Methods for implantable Heart Monitors, which application is assigned to the assignee of the present application, describes an algorithm for distinguishing between pacing terminable and non-pacing terminable atrial and ventricular arrhythmias. An implantable cardiac device may also distinguish between different degrees of atrial fibrillation, e.g., between atrial fibrillation of high disorganization and atrial fibrillation of intermediate organization, to provide different levels of atrial defibrillation therapy depending upon the degree of disorganization. The application of such a function in an atrial defibrillator is described in U.S. Pat. No. 5,549,641 to Ayers, et al.
A limitation of current bradycardia pacemakers and similar implantable cardiac devices is the response of such devices to detected supraventricular tachycardias, such as atrial flutter. Supraventricular tachycardias are rapid heart rates due to a pacemaker anywhere above the ventricular level, i.e., the sinus node, the atria, or the atria ventricular junction. Atrial flutter, in particular, is a rapid regular atrial contraction. As discussed above, a typical bradycardia pacemaker will respond to a supraventricular tachycardia by mode switching such that ventricular pacing does not track the atrial rate. Nevertheless, a rapid supraventricular arrhythmia may induce a ventricular arrhythmia as well. What is desired, therefore, is an implantable pacemaker or other cardiac device which controls a pacemaker to provide an appropriate pacing therapy in response to the detection of a supraventricular tachycardia. In particular, what is desired is an implantable cardiac pacemaker or other device which employs discrimination algorithms to distinguish between different rapid regular atrial arrhythmias (e.g., fast atrial flutter and other atrial flutter) and which provides an appropriate pacing therapy, in either the atria or ventricles, corresponding to the type of arrhythmia identified.